In a typical hot rolling line for producing hot rolled strips, a hot steel slab is rolled into a hot rolled strip by a hot rolling train including a hot roughing rolling mill and a hot finishing rolling mill, and the hot rolled strip is cooled by cooling water while running on a hot runout table composed of a plurality of table rolls, and is then coiled with a coiler, thus obtaining a hot rolled strip coil.
In the hot rolling line, the hot rolled strip runs on the hot runout table in an unstable state on free tension from when the head end of the hot rolled strip passes through the hot rolling train and until when the head end is coiled with the coiler. Therefore, a phenomenon in which the head end of the strip lifts from a hot runout table 50 (pass line) (hereinafter referred to as “jumping”) 51a tends to occur, as shown in FIG. 32(i). When the jumping 51a becomes excessively large, a phenomenon in which the head end of the strip is folded in a direction opposite to the strip running direction (hereinafter referred to as a “head folding defect”) 52a occurs, as shown in FIG. 32(ii).
While the head end of the hot rolled strip similarly runs on the hot runout table 50 on free tension, when the strip running velocity on the downstream side becomes lower than the strip running velocity on the upstream side for some reason (for example, by the influence of cooling water supplied from above), a phenomenon in which the hot rolled strip waves (hereinafter referred to as “waving”) 53a occurs, as shown in FIG. 33(i). When the waving 53a increases in size, a phenomenon in which the waving portion is folded in the direction opposite to the strip running direction (hereinafter referred to as a “strip folding defect”) 54a occurs, as shown in FIG. 33(ii).
From when the head end of the hot rolled strip is wound on the coiler and until when the tail end of the hot rolled strip passes through the hot rolling train, the hot rolled strip runs on the hot runout table in a tensioned state. Therefore, unordinary displacement, such as the above-described waving, will not occur. However, after the tail end of the hot rolled strip passes through the hot rolling train, the hot rolled strip runs again on the hot runout table in an unstable state on free tension. As shown in FIG. 34(i), jumping 51b occurs and the tail end of the strip moves up and down in a waving form. When the jumping 51b excessively increases in sizes, a phenomenon in which the tail end of the strip is folded in the strip running direction (hereinafter referred to as a “tail folding defect) 52b occurs, as shown in FIG. 34(ii). In a manner similar to that in the above-described waving that occurs at the head end of the strip, when the strip running velocity on the downstream side becomes lower than the strip running velocity on the upstream side for some reason, waving 53b also occurs at the tail end of the strip, as shown in FIG. 35(i). When the waving 53b increases in size, a strip folding defect 54b is caused, as shown in FIG. 35(ii).
Recently, the thickness of hot rolled strips has been increasingly reduced according to user demands. On the other hand, the running velocity tends to increase in order to ensure high productivity. The probability that the above-described unordinary displacement (unstable phenomenon), such as jumping or waving, of hot rolled strips on the hot runout table will occur increases as the thickness of the hot rolled strips decreases and as the running velocity increases.
When the jumping 51a and the head folding defect 52a described above occur at the head end of a hot rolled strip, the head end cannot enter between pinch rolls on the upstream side of the coiler, and the hot rolled strip cannot be coiled with the coiler. Moreover, the pinch rolls and the peripheral instruments including the coiler may be damaged by the impact made when a strip portion with the jumping 51a or the head folding defect 52a collides therewith. Even if the hot rolled strip can be coiled with the coiler, a strip portion that is not smoothly wound, that is, a strip portion having the head folding defect 52a or scratches must be removed by cutting in the next process. This pronouncedly lowers the production yield.
When the jumping 51b or the tail folding defect 52b occurs at the tail end of the hot rolled strip, it is difficult to neatly wind the tail end on the coiler. Furthermore, the components of the hot runout table may be damaged depending on the degree of the jumping 51b and the tail folding defect 52b (the condition of jumping or waving). For example, spliters of the hot rolled strip produced in such a case sometimes fall on the hot rolled strip, and make scratches thereon. In this case, even if the hot rolled strip can be coiled with the coiler, a strip portion that is not smoothly wound, that is, a strip portion having the tail folding defect 52b or scratches must be removed by cutting in the next process. This lowers the production yield.
When the waving 53a and 53b and the strip folding defects 54a and 54b occur at the head end and tail end of the hot rolled strip, coiling of the strip may be hindered and the instruments may be damaged, in a manner similar to that in the case in which the jumping 51a and 51b, the head folding defect 52a, and the tail folding defect 52b occur. Since cooling on the hot runout table by cooling water is not uniform in the longitudinal direction of the hot rolled strip, the material of the hot rolled strip is uneven. As a result, a strip portion having the strip folding defects 54a and 54b and the strip portion having quality variations must be removed by cutting, and this pronouncedly lowers the production yield.
As described above, in the production of hot rolled strips, it is quite important, for high productivity and high quality of the hot rolled strips, to cause the hot rolled strips to stably run on the hot runout table by preventing unordinary displacement (unstable running phenomenon).
The above-described unordinary displacement (unstable running phenomenon) of the strips can be reduced to some extent by decreasing the line velocity. However, the reduction in line velocity lowers the productivity of the hot rolled strips. Moreover, since high quality of the strips cannot be ensured, for example, the finishing temperature cannot be ensured, it is difficult to adopt this method.
In order to ensure running stability of hot rolled strips on the hot runout table, the following proposals have been submitted:    (1) Jumping at the head end of a hot rolled strip running on the hot runout table is pushed by spraying horizontal or oblique jets of gas or liquid from nozzles. (Document 1: Japanese Examined Patent Application Publication No. 52-30137)    (2) Water is directly sprayed onto the surface of a hot rolled strip, which is running on the hot runout table, in an obliquely upward direction by spray devices on the upstream side of the hot runout table, and a velocity component of the sprayed water in the strip running direction is set to be higher than the running velocity of the hot rolled strip so that a thrust acts on the hot rolled strip. This prevents jumping or waving at the head end of the hot rolled strip. (Document 2: Japanese Unexamined Patent Application Publication No. 10-118709).    (3) When the head end of a hot rolled strip runs on the hot runout table, water is horizontally sprayed at an angle of approximately 5° to 30° to the strip running direction from spray devices disposed by the side of the hot runout table, thereby preventing jumping that causes a head folding defect at the head end of the hot rolled strip. (Document 3: Japanese Unexamined Patent Application Publication No. 2001-340911).    (4) While the tail end of a hot rolled strip runs on the hot runout table, high-pressure water is directly sprayed onto the surface of the hot rolled strip in the direction opposite to the strip running direction, thereby preventing waving at the tail end. (Document 4: Japanese Unexamined Patent Application Publication No. 11-267732, Document 5: Japanese Unexamined Patent Application Publication No. 2002-192214)